A. Fixed set of tags
B. Predefined document structure
C. User-definable markup language
D. Client-side execution

Solution

C. User-definable markup language

Explanation

GML (Standard Generalized Markup Language) is a meta-markup language that allows users to define their own markup languages and tags. This flexibility is the key feature that distinguishes SGML from HTML. In contrast, HTML (HyperText Markup Language) has a fixed, predefined set of tags